Elevator systems comprise various features to ensure the safe and efficient operation of the system. One such feature is the use of a safety chain which requires that several conditions, necessary for safe operation of an elevator car, are met. The safety chain includes electrical contact switches for elevator safety devices and for closure of elevator and hoistway doors. The electrical contact switches provide verification to an elevator controller, located in the machine room, that the necessary mechanical conditions of the elevator doors are met.
Most modern elevator door systems have safety chains, which are essentially circuits, comprising a relay contact switch that detects closure of all car and hoistway doors, a car door contact switch that monitors the car door, and hoistway door contact switches that monitor the hoistway doors. The safety chain is interfaced with the elevator controller which processes the information received from the safety chain to determine whether it is safe to move the elevator car. Because safety codes require an electromechanical lock, the car door and hoistway door contact switches are integrated into the mechanical door locks.
In the conventional hoistway door and car door locks, the lock typically comprises a door contact switch including a spring loaded contact and a shorting bar. When the doors are mechanically locked, the shorting bar comes into contact with the spring loaded contact and a signal is sent to the elevator controller that the hoistway doors are locked.
When all the safety devices and hoistway door and car door contacts along the safety chain are closed, indicating that all the necessary safety conditions are met, the elevator controls are signaled that it is safe to move.
While these door lock systems are commonly used, they are subject to several potential problems. One such problem involves dirt and buildup of carbon on the electrical contacts causing a "false" signal. This "false" signal is produced by disallowing the shorting of the electrical contact which prevents a signal being sent to the elevator controller that the doors have been mechanically locked. Another potential problem is shorting of the spring loaded contact by an object other than the shorting bar, such as a coin or metal piece. This also causes a "false" signal and may prevent the door lock from mechanically locking. Furthermore, electrical losses at each lock, which are in series, do not allow the signal to be read reliably by the controller. All of these problems may cause conditions which are costly and time consuming to correct.
Therefore, there exists a need for a safety chain which provides a more reliable means of verifying mechanical locking.